/Engine/Source/Runtime/Engine/Classes/Engine/LocalPlayer.h
this happens automatically for all viewports that exist during the initial server connect so it's only necessary to manually call this for viewports created after that if the join fails (because the server was full, for example) all viewports on this client will be disconnected
virtual void SendSplitJoin ( [TArray](API\Runtime\Core\Containers\TArray)< [FString](API\Runtime\Core\Containers\FString) > & Options )